mybatis源码
1、回顾JDBC
1.1 jdbc执行流程
实现代码：
/*第一步，获取连接*/
Connection connection = DriverManager.getConnection(JDBC.URL,JDBC.USER,JDBC.PASSWORD);
/*第二步，预编译SQL*/
PreparedStatement statement = connection.prepareStatment("SELECT * FROM USER");
/*第三步，执行SQL*/
ResultSet resultSet = statement.executeQuery(statement);
/*第四步，获取结果集*/
readResultSet(resultSet);
1.2 SqlSessionFactory & SqlSession
1.2.1 获取SqlSession
1.2.1.1 源码解析
- SqlSessionFactoryBuilder.build();
public class SqlSessionFactoryBuilder {
public SqlSessionFactory build(Reader reader) {
/**调用内部build方法*/
return build(reader, null, null);
}
public SqlSessionFactory build(Reader reader, String environment) {
return build(reader, environment, null);
}
public SqlSessionFactory build(Reader reader, Properties properties) {
return build(reader, null, properties);
}
}
- SqlSessionFactoryBuilder内部build()一个DefaultSqlSessionFactory
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
/**解析XML解析器*/
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
/**parser.parse() 获取Configuration,build一个DefaultSqlSessionFactory*/
return build(parser.parse());
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error building SqlSession.", e);
} finally {
ErrorContext.instance().reset();
try {
reader.close();
} catch (IOException e) {
// Intentionally ignore. Prefer previous error.
}
}
}
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
}
- 获取到SqlSessionFactory后通过SqlSessionFactory获取SqlSession对象
/**SqlSessionFactory对象中的openSession方法最终都会调用openSessionFromDataSource方法*/
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
//通过Configuration获取mybatis的配置信息
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
//结合JDBC的执行流程来看 与数据库相互是statement对象，实际上executor是对于statement的封装，也就是说executor是statement的一个执行器
final Executor executor = configuration.newExecutor(tx, execType);
// 重点！！ 创建了一个DefaultSqlSession对象
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
- 通过以上步骤，我们已经获取到了一个SqlSession，按照JDBC的步骤来说我们应该去执行sql了,结合以下Demo理解
SqlSessionFactory sqlSessionFactory = new SqlSessionFactory();
String resource = "classpath:mybatis-config.xml"
try{
//SqlSessionFactoryBuilder读取配置文件
sqlSessionFactory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsReader(resource));
} catch(IOException e){
e.printStackTrace();
}
//获取sqlSession
SqlSession sqlSession = sqlSessionFactory.openSession()
1.3 MapperProxy
-
到目前为止我们写的.mapper文件还没有使用！！！，下面介绍 MapperPorxy
在mybatis中我的写的dao层的接口其实是MapperProxy在代理，也就是说我们在执行dao层中的方法是，其实是在执行MapperProxy - 我们通过SqlSession从Configuration中找到一个getMapper方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
// mapperRegistry是什么？见名知意 应该是mapper注册机之类的东西，接着往下走看看这个mapperRegister是什么
return mapperRegistry.getMapper(type, sqlSession);
}
- 我们看下MapperRegistry是什么
// MapperRegistry实际上就是一个注册机用来调用MapperProxyFactory工厂的
public class MapperRegistry {
private final Configuration config;
//
private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();
public MapperRegistry(Configuration config) {
this.config = config;
}
@SuppressWarnings("unchecked")
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
// 这里定了一个MapperProxyFactory的工厂
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
//代理工厂的实例 返回了一个mapper的代理对象工厂的实例，这是不是就是我们想要的dao层对象呢？我们继续往下看MapperProxyFactory
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
}
- MapperProxyFactory的作用是什么
/**部分代码*/
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
private final Map<Method, MapperMethodInvoker> methodCache = new ConcurrentHashMap<>();
public MapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}
public Class<T> getMapperInterface() {
return mapperInterface;
}
public Map<Method, MapperMethodInvoker> getMethodCache() {
return methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
//！！！重点来了 这里代理到了我们写的Dao层接口
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}
通过以上的动态代理就可以获取到我们的Dao层
//获取Dao对象
TestDao dao = sqlSession.getMapper(TestDao.class);
有个疑问，我们定义的Dao是接口（Interface）,按理说接口是不能实例对象的，那我们这个对象是怎么得到的呢？
答：建议补习java的动态道理，划重点-->Proxy.newProxyInstance() AOP也是基于动态代理实现的！
1.4 Excutor
- 到这里我们获取到了SqlSession 和 我们的mapper接口，那接下来应该做什么呢？ 没错！ 执行SQL ，我们去看下真正的SQL执行流程
源码交流群 8671577531
下节讲